Kinetics study on the dehydroxylation and phase transformation of Mg3Si2O5(OH)4

Journal of Alloys and Compounds 713 (2017) 180e186

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Kinetics study on the dehydroxylation and phase transformation of Mg3Si2O5(OH)4 Shiwei Zhou a, b, Yonggang Wei a, b, *, Bo Li a, b, Baozhong Ma a, b, Chengyan Wang c, **, Hua Wang a, b a b c

State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming, 650093, China Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming, 650093, China School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing, 100083, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 October 2016 Received in revised form 12 February 2017 Accepted 14 April 2017 Available online 15 April 2017

Determination of the decomposition mechanism and phase transformation of lizardite [Mg3Si2O5(OH)4] within laterite was carried out using thermal analysis kinetics during the heat-treatment process. Nonisothermal kinetic heating experiments at temperatures ranging from room temperature to 1273 K showed two characteristic peaks at approximately 898 K and 1094 K, corresponding to dehydroxylation and a phase transformation, respectively. Generalized master plots were used as a straightforward approach for determining the kinetic models. The AvramieErofeev reaction model was found to provide the best fit to the dehydroxylation process, and diffusion patterns with an apparent activation energy of 219 kJ mol
1 are discussed. The results indicated that dehydroxylation was controlled by a constant nucleation rate and two-dimensional diffusion. In addition, the crystalline transformation of lizardite presented a reaction process-dependent evolution of the apparent activation energy, which decreased rapidly with increasing extent of the conversion. Based on the analyses, the phase transformation of lizardite may belong to reactions complicated by diffusion. © 2017 Elsevier B.V. All rights reserved.

Keywords: Kinetics Dehydroxylation Crystalline transformation Lizardite

1. Introduction Serpentine minerals are hydrous phyllosilicates with an ideal chemical formula Mg3Si2O5(OH)4. The diversity of serpentine minerals is a result of variations in their chemical compositions and layered structures [1], which has led to the wide use of some serpentine minerals in the production of refractories, magnesium oxide, and silicon dioxide. Many unsaturated SieOeSi, OeSieO, and OH
bonds exist in the fault planes of serpentine, and the OH
released from serpentine by a dehydration reaction decreases the effective pressure and instability along these faults; these faults may also be the reason for deep-seated earthquakes [2,3]. Therefore, the dehydroxylation process and phase transformation of serpentine minerals require in-depth research.

* Corresponding author. State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming, 650093, China. ** Corresponding author. School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing, 100083, China. E-mail addresses: [email protected] (Y. Wei), [email protected] (C. Wang). http://dx.doi.org/10.1016/j.jallcom.2017.04.162 0925-8388/© 2017 Elsevier B.V. All rights reserved.

Because of the chemical composition of serpentine and the physicochemical conditions prevailing during its formation, the curvature of its 1:1 layers differs, leading to three distinct varieties [4]: chrysotile, which displays cylindrical layers, antigorite, which is characterized by a corrugated structure, and lizardite, which exhibits a planar structure. Many previous investigations on the thermal behaviour of serpentine minerals have mainly focused on chrysotile and/or antigorite minerals [5e18], whereas studies on the thermal behaviour of lizardite are rare [19e24]. Some scholars have only used thermogravimetric (TG) and/or differential thermogravimetric (DTG) data to investigate the kinetics of serpentine transformation behaviour [17,25,26]. However, differential scanning calorimetry (DSC) data obtained in this study has shown a broad endothermic peak at ~898 K, corresponding to dehydroxylation, and a sharp exothermic peak at ~1094 K, related to the phase transformation. Therefore, both processes should be investigated. For the phase transformation process, the DSC data could be used to study the reaction mechanism and apparent activation energy. As a kind of refractory material, lizardite exists widely in nature and is deemed to be the archetypical model for Mg-rich serpentines

S. Zhou et al. / Journal of Alloys and Compounds 713 (2017) 180e186

with a trioctahedral 1:1 phyllosilicate structure [27]. Thus, the heating changes and kinetic mechanisms of lizardite should be investigated in detail in order to extend its application. As a matter of fact, the thermal behaviours of lizardite are complex processes involving mass and heat transfer phenomena, phase transformation, and possible intermediates. In this work, the kinetics of lizardite thermal behaviour, including dehydroxylation and phase transformation, were studied separately based on DSC data by a combined isoconversional analysis and master plot procedure. This method does not require the use of complicated mathematical procedures, and it can obtain quite accurate results despite the complexity of the process [28]. 2. Experimental methods 2.1. Starting material Samples of lizardite formed from laterite were taken from Yunnan province, China. The crystalline phases of the laterite samples at room temperature (RT) were detected by X-ray powder diffraction, which showed diffraction peaks mainly due to lizardite (Fig. 1). The chemical compositions of the samples are listed in Table 1. The lizardite crystals contained minor amounts of aluminium oxide (1.89 wt%) and iron (9.67 wt%).

181

Table 1 Chemical analysis of the lizardite sample. Component Content wt.%

MgO 31.49

SiO2 37.37

Fe (total) 9.67

Al2O3 1.89

CaO 0.033

Mn 0.083

2.4. Kinetic approach The non-isothermal thermal analysis method was used to study the dehydroxylation mechanism of the lizardite. The general rate equation is given by

da ¼ kðTÞf ðaÞ dt

(1)

The reaction model, f(a), describes the dependence of the extent of the conversion (a) on the reaction progress. The dependence of the process rate on temperature is represented by the rate constant k(T), which is typically expressed using the Arrhenius equation.



E kðTÞ ¼ A exp RT

(2)

Combining Eqs. (1) and (2) yields


 da
E f ðaÞ ¼ A exp RT dt

(3)

2.2. X-ray powder diffraction After heat treating samples at temperatures of 873, 923, 948, 973, 1023, 1073, 1123, 1173, 1273, and 1473 K for 2 h, each sample was subjected to X-ray powder diffraction (XRPD) analysis performed with a Japan Science D/max-R diffractometer (Cu Ka radiation generated at 40 kV and 40 mA). The diffractograms were measured in the 10e80 2q range using a step size of 0.01. 2.3. Thermogravimetric and differential scanning calorimetry analyses TG/DSC analyses were conducted with a NETZSCH STA 449F3 device under a constant flux of nitrogen (75 mL/min N2) to reduce possible oxidation effects. The kinetic studies of the powdered samples were carried out at five different linear heating rates (5, 10, 15, 20, and 30 K min
1).

2.4.1. Isoconversional analysis Isoconversional methods are frequently called “model-free” methods, which do not need to identify the reaction model. Several of the most popular isoconversional methods were used and described as follows. The Friedman method: The Friedman method is the most commonly used differential isoconversional method [29]; it is proposed to extract model-free values of the activation energy (E) from the logarithmic form of Eq. (3). Thus,

"


ln bi

 # da Ea ¼ ln½Af ðaÞ
dt a;i RTa;i

(4)

The KissingereAkahiraeSunose (KAS) method: In the KAS method, the relation between the temperature, Tai, and the heating rate, b, is given by Refs. [30,31].

bi

ln

!

Ta2i

Ea þ const ¼
RTai

(5)

The Starink method: The method proposed by Starink is given as [32].

ln

bi

!

Ta1:92 i


Ea þ const ¼
1:0008 RTai

(6)

The Tang method: A temperature integral relation has been suggested by Tang and is given as [33,34].

ln

bi Ta1:895 i

!


Ea þ const ¼
1:00145 RTai

(7)

where b is the heating rate, given by

Fig. 1. XRD pattern of sample.

b¼

dT ¼ const dt

(8)

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S. Zhou et al. / Journal of Alloys and Compounds 713 (2017) 180e186

A series of runs at different heating rates, i.e., 5, 10, 15, 20, and 30 K min
1, were performed. Then, the value of Ea at a constant a value was determined from the slope of the plot of the left sides of Eqs. (4)e(7) against the inverse of the temperature. 2.4.2. Master plots The z(a) master plots [35] and generalized time master plots [36] are depicted in order to make inferences on possible ratelimiting steps and reaction models. A suitable model is identified as the most appropriate match by comparing these theoretical master plots to the normalized experimental data. The z(a) master plots were derived by combing the differential and integral forms of the reaction models. The integral form of the reaction model can be obtained by the integration of Eq. (3).

gðaÞ ¼

Za 0

da ¼A f ðaÞ

Zt



E dt ¼ Aq exp RT

(9)

0

For constant heating-rate conditions, Eq. (9) is usually rearranged as

gðaÞ ¼

A

b

ZT



E dT exp RT

(10)

0

The temperature integral in Eq. (10) can be replaced with various approximations [37], p(x), as follows:

gðaÞ ¼

  AE pðxÞ expð
xÞ x bR

(11)

where x ¼ E/RT. Combining Eqs. (1) and (11) yields the z(a) function as

zðaÞ ¼ fðaÞgðaÞ ¼




   da pðxÞ Ta2 dt a bTa

(12)

In addition, Eq. (12) is normalized to the reaction rate at a ¼ 50% and the normalized function is given by

  da

dt zðaÞy  a

da dt 0:5




Ta T0:5

2 (13)

Equation (13) is simplified by removing the term in the brackets of Eq. (12) because of its negligible effect on the shape of the z(a) function [35,38]. The generalized time master plots approach is based on the generalized time, q, which can be obtained using Eq. (9).

q¼

Zt



E dt exp RT

da=dq f ðaÞ da=dt expðE=RTa Þ ¼ ¼ ðda=dqÞ0:5 f ð0:5Þ ðda=dtÞ0:5 expðE=RT0:5 Þ

(17)

The resulting experimental data are plotted as a function of a and compared against theoretical master plots. A suitable model is identified as the best match between the experimental and theoretical master plots. 3. Results and discussion 3.1. Sample characterization The XRPD pattern of the laterite sample showed the predominance of lizardite; in addition, the samples contained a small quantity of quartz and magnetite, as shown in Fig. 1. The mass loss values and changes of thermal balance within the lizardite upon heating were estimated based on the TG/DSC curves (Fig. 2). The total release of moisture estimated from the TG curve was approximately 12.0 wt%. This experimental value deviates slightly from that of the theoretical calculation (13.0 wt%), which may be attributed to the presence of impurities or defects within the sample. In addition, the DSC data (Fig. 3) showed a broad endothermic peak in the 853e973 K range, which is related to the dehydroxylation of lizardite [Mg3Si2O5(OH)4/Mg3Si2O7þ2H2O]; a sharp exothermic peak occurs at approximately 1094 K, corresponding to the crystallization of lizardite to the olivine group (Mg3Si2O7/Mg2SiO4þMgSiO3) [39]. 3.2. Kinetics of dehydroxylation According to the aforementioned TG/DSC data, the dehydroxylation of lizardite within laterite occurred in the 853e973 K range. Thus, the non-isothermal thermogravimetric analysis method was used to study the kinetics of the lizardite dehydroxylation reaction. Fig. 4 shows ln(da/dt) as a function of 1/T in the 0.1 < a < 0.9 range. It can be seen that parallel straight lines are obtained. Using Eq. (4), the respective E values were calculated from the slopes of these straight lines and are listed in Table 3. For a ¼ 0.1e0.9, the E values varies only slightly, i.e. they remain practically constant, yielding an average value of 219 kJ mol
1. The dehydroxylation of lizardite can thus be adequately described as single-step kinetics [35]. Additionally, this value is consistent with the earlier findings of Gualtieri, who obtained an apparent activation energy value of

(14)

0

The first derivative of generalized time can be written as

dq ¼ e
E=RT dt

(15)

Combining Eqs. (3) and (15) leads to


 da da E exp ¼ Af ðaÞ ¼ RT dq dt

(16)

Taking a ¼ 50% as a reference, the relationship between the generalized reaction rate and the experimental data for the nonisothermal case can be expressed as

Fig. 2. TG/DSC curves for thermal decomposition of sample (b ¼ 15 Kmin
1in nitrogen atmosphere).

S. Zhou et al. / Journal of Alloys and Compounds 713 (2017) 180e186

183

Table 3 Activation energy obtained by Friedman plot.

Fig. 3. DSC-signals as a function of temperature and heating rate.

Fig. 4. Reaction rate ln(da/dt) as a function of the inverse absolute temperature for five individual DSC runs.

221 kJ mol
1 using isothermal data [40]. To make inferences on possible reaction mechanisms, the normalized experimental data and the theoretical curves of the z(a) master plots calculated for several commonly used reaction models (listed in Table 2) are depicted in Fig. 5. It is evident that the dehydroxylation reaction model of lizardite under nitrogen atmosphere is slightly affected by the heating rate. The experimental

Extent of conversion, a

E/kJ mol
1

Regression

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

223 225 224 223 222 217 211 210 212

0.99731 0.99687 0.99663 0.99641 0.99500 0.99560 0.99120 0.98736 0.98420

Average E/kJ mol
1

219

data presented in Fig. 5 are in good agreement with the AvramieErofeev (A3) reaction model. In addition, the maxima zmax(a) values of the individual normalized experimental curves are close to the theoretical value of ap ¼ 0.632 [41], which further confirms the accuracy of the A3 model for the dehydroxylation of lizardite. The AvramieErofeev coefficient is given as n ¼ a þ b, where a is the number of steps involved in nucleus formation and b is the number of dimensions in which the nuclei grow [42]. The AvramieErofeev coefficient (n ¼ 3), number of steps (a ¼ 1), and number of dimensions (b ¼ 2) obtained in this work for the dehydroxylation of lizardite indicate that the rate-limiting step of the reaction should be a constant nucleation rate and twodimensional diffusion [42]. Perillat et al. reported n ¼ 2 for antigorite decomposition and suggested a reaction mechanism with a rate-limiting step determined by instantaneous nucleation and interface [43]. Brindley et al. suggested a lizardite dehydroxylation reaction controlled by diffusion [44]. Gualtieri et al. showed that the rate-limiting step of the dehydroxylation reaction of lizardite is a decelerating nucleation rate and two-dimensional diffusion [40]. Lizardite has an idealized formula of Mg3Si2O5(OH)4, and it possesses a planar structure, shown in Fig. 6. The OH
groups exist in the lizardite structure in two positions: inner-surface OH
groups are located on top of the trioctahedral sheet, whereas inner OH
groups are located between the trioctahedral and tetrahedral sheets [45]. Based on the kinetics analysis and the structure of lizardite, the dehydroxylation reaction model of lizardite during heating can be divided into three types [40]: (a) H-bond bridging breaks and then combines with an adjacent hydroxyl to form a further HeO bond; also, the MgeO bond in the Mgewater molecule clusters breaks and the water molecules diffuse along the twodimensional interlayer region; (b) MgeO bonds in Mgehydroxyl clusters and H-bond bridging break and the dissociated hydroxyls diffuse along the two-dimensional interlayer region; (c) H-bond bridging breaks and then the hydrogen diffuses along the twodimensional interlayer region. Fig. 7 shows the three different diffusion patterns and their corresponding activation energies. The calculated activation energies of the different models suggest that

Table 2 Differential f(a) and integral g(a) functions of some widely used kinetic models in the solid-state kinetics. Reaction model

Code

f(a)

g(a)

One-dimensional diffusion

D1

1=ð2aÞ

a2

Two-dimensional diffusion

D2

ð1
aÞlnð1
aÞ þ a

AvramieErofeev eq., n ¼ 2

½
lnð1
aÞ
1

A2

AvramieErofeev eq., n ¼ 3

2ð1
aÞ½
lnð1
aÞ1=2

½
lnð1
aÞ1=2

A3

AvramieErofeev eq., n ¼ 4

3ð1
aÞ½
lnð1
aÞ2=3

½
lnð1
aÞ1=3

A4

4ð1
aÞ½
lnð1
aÞ3=4

½
lnð1
aÞ1=4

Phase-boundary-controlled reaction (contracting cylinder)

R2

1=2

2ð1
aÞ

½1
ð1
aÞ1=2 

Phase-boundary-controlled reaction (contracting sphere)

R3

Mampel (first order)

F1

3ð1
aÞ2=3 1
a

½1
ð1
aÞ1=3 
lnð1
aÞ

184

S. Zhou et al. / Journal of Alloys and Compounds 713 (2017) 180e186

Fig. 5. Comparison between theoretical z(a) and experimental non-isothermal DSC data of the lizardite dehydroxylation under N2 atmosphere.

the energy of model (c), with its diffusion of hydrogen atoms, is closer to the experimental value (219 kJ mol
1). 3.3. Kinetics of phase transformation Fig. 2 shows a sharp exothermic peak at approximately 1094 K, which corresponds to the phase transformation of lizardite. Based on the literature [40], when the heating rate is higher than 30 K min
1, the metastable talc-like phase disappears; therefore, the process of phase transformation might be affected by a higher heating rate. In the present study, DSC data obtained from four different heating rates (5, 10, 15, and 20 K min
1) were used to investigate the kinetic process of phase transformation. The activation energy of the process was obtained using the KAS (Eq. (5)), Starink (Eq. (6)), and Tang (Eq. (7)) methods. Table 4 lists the apparent activation energies, Ea, as a function of a for the phase transformation of lizardite. The results of the three methods show that Ea decreases as a increases (Table 4). It has been proposed that the activation energy computed for a multi-step process might vary with a [50]. Elder and Dowdy concluded that the isoconversional methods are applicable to the investigation of multi-step processes [51e55]. In addition, Vyazovkin and Lesnikovich reported that the significant dependence of Ea on a could help not only to disclose the complexity of a process but also to identify its kinetic scheme [56]. Furthermore, based on the simulated data, the shapes of the dependence of Ea on a have been identified for various processes [56e59]. The most characteristic variations have been summarized by scholars [60], they

Fig. 7. The structure of lizardite and the mechanism of dehydroxylation. The grey tetrahedral represents the silica layer. Note the activation energy in each step is referenced to the results of other studies by scholars [46e49].

indicated that a decreasing dependence of Ea on a is found for reaction complicated by diffusion. Therefore, based on the characteristics of Ea, the phase transformation of lizardite could be described as reactions complicated by diffusion. Although the shape of the dependence of Ea on a may not identify the kinetic scheme of the phase transformation of lizardite unequivocally, it may help shed light on further investigations to some extent. 3.4. Process analysis of phase transformation The main original phases in the investigated specimens were lizardite, quartz, and magnetite (Fig. 1). Fig. 8 shows the phase transformation of the original and the newly formed phases in the investigated system during heat treatment. Below 873 K, qualitative phase analysis showed the coexistence of crystals of lizardite,

Table 4 Activation energy of phase transformation of serpentine at different extent of conversion (a) for different iso-conversional methods. Degree of conversion (a)

Fig. 6. Structure of lizardite-1T.

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

E/K mol
1 KAS method

Tang method

Starink method

1319 1219 1168 1074 1016 957 922 832 743

1322 1222 1171 1077 1019 959 924 834 745

1321 1221 1170 1076 1018 958 924 833 746

S. Zhou et al. / Journal of Alloys and Compounds 713 (2017) 180e186

185

Fig. 8. XRPD patterns of the lizardite after heat treatment (a) 873e1023 K; (b) 1073e1473 K.

hematite, and quartz. As the temperature increased to 923 K, the lizardite phases disappeared and no newly formed phases were observed. Forsterite diffraction peaks can be observed only at T > 948 K. These findings indicate that the lizardite in the investigated sample decomposed completely at approximately 948 K, corresponding to the formation of forsterite. In addition, the intensity of the forsterite diffraction peaks increased with increasing temperature to 1273 K; the appearance of enstatite occurred with a weak diffraction peak after the formation of forsterite, when

Fig. 9. Evolution of the integrated intensities of the original and newly formed phases in the investigated system during the non-isothermal runs for lizardite.

lizardite was completely decomposed at T > 1073 K. Therefore, sharp exothermic peaks with maximum of approximately 1094 K on the DSC curves indicate overlapping processes of forsterite enlargement and recrystallization [25]. As the temperature increased to 1473 K, the diffraction peaks of enstatite were significantly enhanced, accompanied by weakening of the forsterite diffraction peaks. This may be attributed to the reaction of forsterite with a Si-rich amorphous segregation product (SiO2) to form enstatite phases (Mg2SiO4þSiO2/2MgSiO3). Talc-like phases were not observed in the XRPD patterns. Based on the literature [40], a metastable talc-like phase appears in minor amounts in the early stages of the dehydroxylation of lizardite at heating rates slower than 30 K min
1. In the present study, however, the sample was placed into the furnace at the actual set temperature, i.e., without being slowly heated, resulting in rapid dehydroxylation, which is not conducive to the formation of the talc phase. Fig. 9 depicts the evolution of the integrated intensities of the main diffraction peaks relative to the phases observed during the phase transformation of the lizardite sample at high temperature (T > 873 K). The first stage is the dehydroxylation of the lizardite phase (T < 873 K), followed by the enlargement and recrystallization of forsterite above 948 K and then by the crystallization of enstatite. The experimental results in this study disagree with those in previous investigations reported by Weber and Greer [15]. In their systematic differential thermal analysis research, the dehydroxylation of lizardite presented an activation energy of 52e319 kJ mol
1), with an average Avrami-Erofeev coefficient n of 0.63 [15]. Brindley et al. [44] and Gualtieri et al. [40] reported apparent activation energies for the dehydroxylation of lizardite of 284 kJ mol
1 and 221 kJ mol
1, respectively; these two values are

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S. Zhou et al. / Journal of Alloys and Compounds 713 (2017) 180e186

relatively close to the results reported in this study. In general, the activation energy of lizardite in this study is lower than that reported in the literature. In addition, there are obvious differences between different scholars' research results. The reasons are mainly attributed to the various specimens used in these studies. The interplay of factors, including the chemical composition, crystal defects, crystal size, and experimental setup, would affect the final experimental results [40]. Furthermore, the phase transformation at elevated temperature would also be affected by these factors. Thus, lizardite comprising various chemical compositions would exhibit different mechanisms of dehydroxylation and phase transformation. 4. Conclusions In this work, the mechanisms of dehydroxylation and phase transformation of lizardite within laterite were investigated in detail by the isoconversional method. The TG/DSC results confirmed that the lizardite had two characteristic peaks, corresponding to the dehydroxylation of lizardite (endothermic peak) and phase transformation (exothermic peak), respectively. The dehydroxylation of lizardite can be described as single-step kinetics, yielding an apparent activation energy of 219 kJ mol
1; the dehydroxylation reaction model under nitrogen atmosphere, which is in accordance with the AvramieErofeev reaction model, A3, was slightly affected by the heating rate. At heating rates higher than 30 K min
1, the phase transformation might be affected. The DSC data obtained from four different heating rates (5, 10, 15, and 20 K min
1) were used to investigate the kinetic process of phase transformation. The results of the isoconversional methods showed that Ea decreased rapidly with increasing a. The process should be described as multiple-step kinetics on the basis of the reaction process, a. The XRPD results revealed that the lizardite phase disappeared when the temperature increased to 923 K, and no newly formed phases were observed. In addition, forsterite diffraction peaks could be observed only at T > 948 K. Acknowledgments Financial support for this study was provided by the National Natural Science Foundation of China (Project Nos. U1302274 and 51304091), the Candidate Talents Training Fund of Yunnan Province (2012HB009), and the Analysis and Testing Foundation of Kunming University of Science and Technology (20150978). References [1] A.E. Balan, M. Saitta, F. Mauri, C. Lemaire, F. Guyot, Am. Mineral. 87 (2002) 1286e1290. [2] P. Ulmer, V. Trommsdorff, Science 268 (1995) 858e861. [3] M. Sawai, I. Katayama, A. Hamada, M. Maeda, S. Nakashima, Phys. Chem. Miner. 40 (2013) 319e330. [4] F.J. Wicks, D. O'Hanley, Reviews in Mineralogy, vol. 19, Mineralogical Society of America, Washington DC, 1988, pp. 91e167. [5] E. Aruja, An X-ray Study of Silicates, Chrysotile, Antigorite, Gumbelite, Ph.D. thesis, University of Cambridge, England, 1943. [6] N.L. Bowen, O.F. Tuttle, Bull. Geol. Soc. Am. 60 (1949) 439e460. [7] G.W. Brindley, S.Z. Ali, Acta Crystallogr. 3 (1950) 25e30. [8] A. Cattaneo, A.F. Gualtieri, G. Artioli, Phys. Chem. Min. 30 (2003) 177e183. [9] A.F. Gualtieri, C. Cavenati, I. Zanatto, M. Meloni, G. Elmi, M.L. Gualtieri, J. Hazard. Mater 152 (2008) 563e570. [10] P.A. Candela, C.D. Crummet, D.J. Earnest, M.R. Frank, A.G. Wylie, Am. Mineral.

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